A mosquito's secret weapon: a light touch and strong wings

Scientists have found the key to mosquitoes' stealth takeoffs:
They barely push off when making a fast getaway, but instead rely on strong and
rapid wing beats to quickly get aloft without anyone noticing.

The technique is in stark contrast to other insects, like flies,
that push off first and then start beating their wings frantically, often
tumbling uncontrollably in the process. That strong pushoff gives away their
location, however, allowing us to swat them more easily than we can swat a
mosquito.

"Mosquitoes take off mostly with their wings and push off
with their legs very, very lightly, or maybe not at all," said Sofia
Chang, a University of California, Berkeley graduate student who wrangled and
fed malarial mosquitoes in order to study their takeoffs. "If they were to
push off a lot more with their legs, they wouldn't have to produce as much lift
with their wings. But if they lift just with their wings, you won't feel them
coming off your skin."

Mosquitoes are able to make these stealthy takeoffs with an
empty belly or one filled with a blood meal, which nearly doubles their weight,
she said.

Working in the laboratory of Florian Muijres at Wageningen
University in the Netherlands, Chang cycled through 600 mosquitoes as the team
perfected its setup to film mosquito takeoffs with three high-speed cameras
shooting at 125,000 frames per second. Chang initially fed them blood from her
own arm before Wageningen University entomologist Jeoren Spitzen lent the
project his entomological expertise and equipment to feed them artificially.

Her team used the mosquito species Anopheles coluzzii -- which
can carry malaria but was kept sterile during the experiment -- in hopes of
finding clues to flight maneuvers that could be used against them.

"These studies may also give tips about how to build very,
very small robots. That is a field where miniaturization is a Holy Grail,"
Chang said.

She and her colleagues, including UC Berkeley integrative
biology professors Robert Dudley and Mimi Koehl, were primarily interested in
how insects alter their takeoffs when carrying extra weight, like a blood meal. The high-speed cameras captured stark silhouettes of the mosquitoes and their
beating wings, which Muijres was able to turn into three-dimensional renderings
of the wingbeats to help calculate lift and other aerodynamic forces. Of the
many mosquitoes filmed, 63 videos were analyzed in the study: 32 of blood-laden
mosquitoes, 31 unfed.

Chang, Muijres and Wouter van Veen were surprised to find that
the mosquitoes began beating their wings about 30 milliseconds before liftoff
using an extraordinarily high -- and annoyingly whining -- wing-beat frequency
of about 600 beats per second. Other similarly sized insects beat their wings
about 200 times per second.

They also discovered that the insects took advantage of their
exceptionally long legs to extend them gently and push down slowly over the 30
milliseconds before takeoff, lifting free with barely a rebound. When Chang and
Muijres calculated the wings' contribution to the takeoff, they were impressed
that they contributed at least 60 percent of the force -- and possibly all of
the force -- needed to lift a well-fed mosquito off the skin.

"Instead of going fast, they take their time, but they
accelerate the entire time so that they reach a final velocity pretty much the
same as fruit flies," Chang said. "That is something that might be
unique to mosquitoes, and maybe even unique to blood feeders."

Fruit flies, the team found, exerted almost four times the force
exerted by mosquitoes during takeoff. Coauthor Bart Biemans compared the
anatomy of the mosquitoes' and fruit flies' leg muscles and found that the
mosquitoes were missing the quick-kick muscles that present in the fruit flies,
presumably "because [mosquitoes] need to produce a lower force at
pushoff," Muijres said.

"These videos revealed that female mosquitoes carrying
blood meals generate the extra lift they need to take off with such heavy loads
by sweeping their wings across a greater distance during each wingbeat than do
mosquitoes that are not carrying loads," Koehl noted.

The team next plans to look at mosquito landings, which are
equally stealthy, and compare them to takeoffs and landings of other
blood-sucking insects and non-blood-sucking mosquitoes, to determine if they
exert a similarly light touch.

The work was supported by the National Science Foundation
(DGE-0903711) and the Wageningen Institute of Animal Sciences. Chang and her
colleagues from UC Berkeley and Wageningen University in the Netherlands report their findings in a paper published Oct. 18 in the Journal
of Experimental Biology.